1. Field of the Invention
The present invention relates to three-phase alternators and more particularly to three-phase alternators used for charging the battery of automobile vehicles.
2. Discussion of the Related Art
FIG. 1 schematically represents a general diagram of a three-phase alternator used for charging an automobile battery. The alternator includes three induced windings L1, L2, L3 interconnected through one of their terminals and an induction coil L. The current in the induction coil L is set by a regulator (REG) that takes into account the current in the load in a way not represented. The output terminals S1, S2, S3 of the alternator are connected through diodes d1, d2, d3 to a first input of the regulator to feed it, and through a rectifying bridge to terminals S.sup.+ and S.sup.- that are the positive and negative terminals of the battery B of the automobile vehicle and that are connected to various load circuits of the automobile vehicle. The rectifier includes diodes D1, D2, D3 connected by their anodes to the respective outputs S1, S2, S3 of the alternator and by their common cathodes to terminal S.sup.+ (through a switch 1 switched on by the switch key), and includes diodes D4, D5, D6 that are connected by their cathodes to the respective outputs S1, S2, S3 and by their common anodes to terminal S.sup.-.
A load indicator 2 is connected through an insulation diode 3 between terminal S.sup.+ and the positive power supply of the regulator. The indicator 2 lights up when the voltage across the regulator becomes lower than the voltage of the battery, that is, in case of trouble or when the motor does not run although the contact is turned on.
The regulator operates to modulate the power provided by the alternator as a function of the required power. However, if the required power across terminals S.sup.+, S.sup.- abruptly drops, for example because of a defective contact of the battery clips or because the lights of the vehicle are switched off, the regulator does not immediately operate and, during a period that is for example, approximately a few hundred milliseconds, the voltage across terminals S1, S2, S3, therefore across terminals S.sup.+ and S.sup.-, abruptly increases. Indeed, the power supply provided by the alternator did not change whereas the current abruptly dropped. This kind of voltage surge is commonly referred to as a "load dump". In the case of conventional automobile equipments, this phenomenon is not a significant problem. However, nowadays, the electric power available in automobile vehicles keeps increasing, and the trend is to incorporate an increasingly large number of active electronic components, realized in the form of integrated circuits. Such integrated circuits are very sensitive to overvoltage phenomena and to temporary overvoltage associated with the load dump, that can reach about 100 volts and may destroy the electronic components mounted on the vehicle, as well as in the regulator.
Various solutions have been proposed in the prior art to solve this problem.
A first known solution, illustrated in FIG. 2, consists in replacing each diode D1-D6 by an avalanche diode Z1-Z6 having a breakdown voltage higher than the maximum voltage across battery B. For the sake of safety, this breakdown threshold must be chosen substantially higher than the voltage of the battery. Thus, for a 12-volt battery, breakdown thresholds of approximately 30 volts are commonly chosen. An overvoltage occurring at one of terminals S1, S2, S3 is therefore clipped by one of the diodes.
A second conventional solution, illustrated in FIG. 3, consists in disposing an avalanche diode Z in parallel with battery B. This avalanche diode must also have a clipping breakdown threshold of approximately 30 volts.
The major drawback of these two solutions using clipping diodes is the dissipation problem due to the fact that the energy during the load dump phase is relatively high (approximately 100 joules), and substantially corresponds to the energy level of the load that has been abruptly interrupted. This energy is dissipated in the clipping diodes through which a high current will subsequently flow. This situation makes it necessary to use large-size diodes and/or to use several diodes in parallel and to associate other power dissipation heat sinks with these diodes. Thus, these solutions require the use of large-surface components associated with casings that are to be specifically mounted.
A third approach is described in U.S. Pat. No. 3,488,560. FIG. 2 of this patent is substantially similarly reproduced in the attached FIG. 4 with modifications in order to render its representation comparable with FIGS. 1-3. In this third solution, the load dump protection circuit is formed by three thyristors T1, T2, T3 that are connected between each output S7, S2, S3 of the alternator and terminal S.sup.-. The gates of thyristors T1, T2, T3 are connected through resistors to the anode of an avalanche diode 5 whose cathode is connected to terminal S.sup.+.
At the occurrence of a load dump, the voltage between terminals S.sup.+ and S.sup.- rapidly increases. This voltage increase causes diode 5 to go to avalanche mode, and therefore causes the current to flow through the gates of thyristors T1, T2, T3. The thyristor having its anode connected to the most positive terminal amongst terminals S1, S2, S3 at the considered time starts conducting and the overvoltage is transmitted back into the other windings by at least one of diodes D4, D5, D6. Then, the thyristor stops conducting when its anode is no longer connected to a positive voltage, and the protection device is blocked if the overvoltage has ceased. Otherwise, another thyristor amongst thyristors T1, T2, T3 (the thyristor that is connected to the most positive terminal S1, S2, S3) relays the preceding thyristor.
An advantage of this third approach is the possibility of using thyristors which, when they are conductive, have a very low voltage drop at their terminals, dissipate little energy and thus can be small-size components that need not be mounted on heat sinks.
However, this third approach has two major drawbacks.
The first drawback of this approach lies in the complexity of its implementation, and the need for using and connecting a plurality of individual components, namely, at least one first component integrating the three thyristors T1, T2, T3 and a second component corresponding to diode 5 and, also, if necessary, additional components for the resistors.
The second drawback is that this approach does not protect the regulator against the mains overvoltages or, if it does, this protection impairs the operation of the circuit. Indeed, regardless of the load dump problems, the electric network of a vehicle may generate many perturbations resulting, for example, from ignition between an ignition coil and the spark plugs or from load variations. These phenomena generate short lasting pulses (a few microseconds) that can reach 300 volts. Such parasitic energy pulses remain however sufficiently energetic to damage the components mounted on the automobile vehicle and may be transmitted to the regulator through diode 3 and indicator 2, thus damaging the electronic circuits of the regulator. Such overvoltages are not clipped by the circuit of FIG. 4 in which diodes D1, D2, D3 block these overvoltages and prevent them from being eliminated by thyristors T1, T2, T3. Furthermore, these overvoltages cannot be eliminated by diode 5 and the gate-cathode path of thyristors T1, T2, T3 because of the presence of high series resistances.